The invention relates to an electrical cable having at least one wire including a conductor and an insulation. The insulation surrounds said conductor and comprises at least two insulation layers. Furthermore, the invention also relates to a method for making such an electrical cable.
In particular the invention addresses the problem of making an electrical cable which is lightweight, cost effective and which can still fulfill certain properties when exposed to a burn test where the electrical cable is exposed to a very high temperature under operation. In particular, the insulation should be lightweight and cost effective and should form hard ashes when exposed to the burn test in order to maintain good insulation properties during a high temperature at least over a predetermined period of time.
The electrical cable according to the invention can be used as a power transmission cable or a communication transmission cable depending on the purpose of use.
When making an electrical cable having at least one wire including a conductor and an insulation surrounding said conductor and when combining said wires to a cable it is very important to make sure that the insulation fulfills certain mechanical and electrical properties. For example, the insulation materials need to be selected such that environmental influences like very low or very high temperatures or humidity do not change the insulation properties to such an extent that a short circuit would arise.
Furthermore, in buildings or installations where increased safety requirements must be satisfied the cables must be fire-resistant. Every country has established certain industrial standards which must be fulfilled by such cables in this respect. For example, according to the German Industrial Standard (DIN Deutsche Industrie-Norm) 4102 Section 12 cables and wires must sustain temperatures up to 1000xc2x0 C. over a period of up to 90 minutes according to a unit temperature time curve. During this time a voltage of 400 Volt is applied to the cables and wires and the cables and wires only pass this burn test if no short circuit or conductor interruption occurs.
In order to provide cables and wires with appropriate insulation properties conventionally conductors have been provided with a double insulation consisting of at least two insulation layers. As shown in FIG. 1, a conductor 7 is surrounded by a first insulation layer 8 and a second insulation layer 9. Typically the first insulation layer 8 consists of a mica tape which is wound on said conductor 7. the second insulation layer 9 may consist of EPR (ethylene-propylene-rubber). If it is, for example, required that the wire continues an operation over 30 minutes or longer when subjected to the burn test, this will require an insulation thickness of a wire with a cross-section of 1.5 mm2 including a mica tape to be 1.15 mm (thickness of the mica tape 0.15 mm thickness of the EPR-insulation 1.0 mm). Since a large insulation thickness is required and since mica tape is quite an expensive material this type of cable is comparatively heavy and expensive. For example, if such type of cables are supplied in great length, e.g. on a spool, then the comparatively large insulation thickness will limit the length that can be supplied on the spool. On the other hand, if the cables are heavy then e.g. the masts needed for holding cables during land line operation need to be very stable and therefore the installation costs also rise.
Another type of a cable K is shown in FIG. 2 and also comprises a double insulation arrangement consisting of a first insulation 8xe2x80x2 and a second insulation layer 9. Both insulation layers 8xe2x80x2, 9 comprise EPR or a silicone rubber compound. Some cables having the construction of FIG. 2 also comprise silicate or even mica as part of the silicone rubber compound. This type of insulation is thicker than the one shown in FIG. 1 and is also cost intensive.
Finally there are also known cables K according to FIG. 3 where the conductor 7 is surrounded by a single layer insulation consisting of hardgrade-EPR. Hardgrade-EPR is a material which has only recently attracted attention and the properties of hardgrade-EPR have been standardized regarding insulation properties, strength etc. For example, the mechanical and electrical properties of hardgrade EPR are defined in IEC 60502.
Furthermore, it may be noted that the mica tape is very cost intensive and it also requires a complicated manufacturing process since the mica tape has to be spun (wound) on the conductor.
FIGS. 4, 5 show conventional constructions of cables comprising a plurality of cores each having core construction according to FIGS. 1, 2. In FIG. 4 the wires are embedded in a common inner sheath 10 which is preferably a fire resistant and halogen free component. Over the inner sheath 10 there is provided a outer sheath coating or cover 11, for example, according to DIN VDE 0266. Conductor 7 consists, for example, of a copper conductor according to DIN VDE 0295 Class 1 or 2, the mica tape consists, for example, of phlogopit and the insulation 9 is a rubber mixture on the basis of EPR according to DIN VDE 0207 E Section 23 mixture type Hl1.
In FIG. 5 a further concentric conductor 12 is provided under the outer sheath 11 and over the common inner sheath 10. The concentric conductor 11 comprises copper filaments including a copper transverse helix.
Constructions as in FIGS. 4, 5 are also possible for the core construction shown in FIG. 3
G 91 16 636.5 describes the burn test for cables according to DIN 4102, Section 12. The fire resistant electrical cable comprises two mica tapes wherein a thin layer of a high temperature resistant hard ash forming silicone rubber adhesive is arranged between said two mica tapes.
G 89 07 116.6 describes a medium voltage or high voltage cable comprising a bandage made of a mica paper band impregnated with silicone resin. An outer conducting layer is also surrounded by a bandage consisting of bands made of mica.
DE 31 379 56 C2 concerns a fire resistant electrical cable having an insulation on a conductor which consists of a polyvinylchloride-mixture. The conductor can also be surrounded by a common layer of silicone rubber. It is described here that at high temperatures the silicone rubber disintegrates and forms powder ashes through which an outer metal layer is held together. The metal layer is a kind of pipe that holds together the ashes in the burn tests.
DE 29 151 88 C2 describes an electrical cable having an insulation consisting of cross-linked polyethylene.
DE 20 51 192 describes a fire resistant electrical cable having an insulation layer and/or an outer layer consisting of magnesium carbonate, chloride and antimon trioxide. The basic component is polyvinylchloride. Several mixtures for the insulation layers are analyzed such as PVC-softeners, stabilizing agents, lubrication means and calcinated kaolin. In particular, ethylene-propylene-rubber (EPR) is used. Only a single insulation layer is mentioned and investigated.
DE 26 59 5415 describes an electrical cable having an insulation made of silicone rubber. A stripe of a polymer/metal-laminate is formed on a conductor and a fire resistant polymer mixture is extruded as outer layer on the surface of said laminate. Therefore, here a double insulation is used.
DE 30 07 341 A1 describes an insulation layer consisting of a mixture of minerals, e.g. silicate or mica. The insulation layer also consists of a binding agent which at least over a predetermined time does not melt in a burn test. An EPR layer is extruded on the mica tape similarly as in the above described FIG. 1. Furthermore, such type of cable is also disclosed in DE 28 10 986.6.
DE 41 32 390 A1 describes an electrical cable having two layers of mica and an outer insulation of an extruded polymer. Furthermore, a high temperature resistant hard ash forming silicone rubber adhesive is used.
DE 44 37 596 A1 describes the use of a hard ash forming silicone rubber fire resistant mixture which contains a silicone compound at least a metal oxide and/or a precursor of said metal oxide and other additives. In particular, it is described that an insulation made of ethylene-propylene-diene-terpolymers (EPDM)-rubber is used. A further insulation layer consists of a non-burnable mineral material such as silicate, glass and hard ash forming silicone rubber. Therefore, this document describes a double insulation made of EPDM and silicate.
DE 28 00 688 C2 describes the use of an EPR-rubber as an outer coating for a cable.
DE 32 28 119 A1 describes a fire resistant cable having conductor insulations consisting of thermal plastic halide free fire resistant polymer mixtures. Over the conductor insulation a foil of glimmer paper is applied.
As explained above, several types of single or double insulation constructions have been used for insulating the core conductor in cable constructions. However, these insulations consist of mica and EPR or EPDM insulations and therefore are heavyweight and cost intensive due to the large thickness of insulation required.
The present invention aims at avoiding these disadvantages of the prior art. In particular, the object of the present invention is to provide an electrical cable and a manufacturing method therefore such that the electrical cable is lightweight and cost effective.
This object is solved by an electrical cable (claim 1) having at least one core including a conductor and an insulation surrounding said conductor and comprising at least two insulation layers, characterized in that wherein a first one of said layers comprises a silicone rubber compound and a second one of said layers comprises an ethylene(C2)-alkylene (Cx)-copolymer or terpolymer mixture adapted to have properties corresponding to those of a hardgrade-ethylene-propylene-rubber (H-EPR).
Furthermore, this object is solved by a method (claim 17) for making an electrical cable, comprising the following steps: providing a conductor; forming an insulation comprising at least a first insulation layer and a second insulation layer on said conductor; wherein in said step b) a silicone rubber compound layer is formed as said first insulation layer; in said step b) a layer of an ethylene(C2)-alkylene (Cx)-copolymer or terpolymer mixture is formed as said second insulation layer; wherein said ethylene(C2)-alkylene(Cx)-copolymer or terpolymer mixture is provided to have properties corresponding to those of a hardgrade-ethylene-propylene-rubber (H-EPR).
According to the invention, one of the two layers provided on the conductor does not comprise EPR or EPDM as explained above, but it includes an ethylene-alkylene-copolymer or terpolymer mixture which has properties corresponding to those of hardgrade-ethylene-propylene-rubber (hardgrade-EPR).
Whilst according to the invention a preferred material is hardgrade-EPR, the invention comprises one insulation layer which consists in general of an ethylene-alkylene-copolymer or terpolymer mixture whose mixing ratio has been adapted such that the corresponding properties of hardgrade-EPR are achieved. The properties which are achieved are the defined properties regarding the insulation properties and electrical properties.
Preferably (claim 4), the second layer can comprise an ethylene-propylene-copolymer or terpolymer mixture, an ethylene-hexene-copolymer or terpolymer mixture or an ethylene-octene-copolymer or terpolymer mixture.
Preferably (claim 5), the first layer is arranged on said conductor and said second layer made of the ethylene-alkylene-copolymer or terpolymer mixture in arranged on said first layer. However, a different cable construction (claim 6) may preferably comprise the second layer on the conductor 1 and the first layer on the second layer.
Preferably (claim 3), the silicone rubber compound comprises a hard ash forming silicone rubber used for the first layer.
Such a silicone compound (claim 10) preferably forms hard ashes during a burn test process.
Preferably (claim 9), the electrical cable constructions according to the invention have properties which allow the cable to conform with the burn test according to the German DIN standard Din 4102 Section 12.
Preferably (claim 11), the electrical cable can comprise a plurality of cores, a sheath surrounding said plurality of cores and an outer coating provided on said sheath. It is also possible (claim 12) that a further conductor is provided under said outer sheath. Preferably (claim 13), the further conductor comprises a plurality of copper filaments.
A particularly advantageous use of the inventive electrical cable (claim 14, 15) is as a communication cable or as a power cable.
Preferably (claim 23), said first layer and said second layer are formed on the conductor by means of an extrusion step. This considerably facilitates the manufacture of the inventive electrical cable.
Preferably (claim 24), the first and second layer are extruded on the respective conductor simultaneously. This can substantially reduce the manufacturing time.
Further advantageous embodiments and improvements of the invention are described in the dependent claims. Furthermore, the invention can comprise embodiments which consist of features which have been described and/or claimed separately in the description and the claims.
Hereinafter, embodiments of the invention will be described with reference to the drawings. It should be noted that the invention is not limited to these embodiments and that the described embodiments only constitute what the inventors presently conceive as best mode of the invention.